Light-emitting diodes (LEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. However, LED devices typically have a highly reflective back electrode to enhance the output of emitted light through one side of the LED device. This highly reflective back electrode also reflects ambient light, thereby reducing the ambient contrast ratio of the LED device. As is known in the prior art, circular polarizers can greatly reduce the reflected ambient light, but such circular polarizers are expensive.
Significant portions of emitted light may also be trapped in LED devices. Scattering layers may be employed to improve the light emission of LED devices, but may inhibit the effectiveness of circular polarizers, and do not selectively absorb ambient light. Chou (International Publication Number WO 02/37580 A1) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124 A1), e.g., taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers in an OLED or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at higher than the critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The light-emitting efficiency of the OLED device is thereby improved, but the ambient contrast is not significantly changed.
One prior-art approach to improving LED device contrast is to employ a black matrix in all non-emitting areas of an LED device, as described, for example in U.S. Pat. No. 6,936,960 entitled “OLED Displays having Improved Contrast” by Cok. The black matrix absorbs the fraction of ambient light incident upon the device between the light-emitting areas, without absorbing emitted light, thereby improving the contrast of the OLED. Generally, it is preferred to maximize the light-emitting area in an LED device to reduce the current density in the light-emitting materials and extend the lifetime of the LED. However, this reduces the amount of area available for a black matrix, thereby increasing the amount of ambient light reflected from the LED back electrode and reducing the contrast of a top-emitting LED device.
Other techniques for reducing ambient light reflection include the use of contrast enhancing films. For example, WO 2005/059636 describes a film having a plurality of waveguides separated by interstitial areas formed as narrowing recesses coated with a reflective layer. However, such a design requires small, high-precision features. Moreover, any imperfection in the reflective layer reduces the absorption of the ambient light and ambient contrast ratio of the device.
There is still a need, therefore, for an improved light-emitting diode device structure that increases the light output and ambient contrast ratio of an LED device.